1. Field of the Invention
This invention relates to Doppler velocity measurements and in particular to the use of a Doppler velocity profiler for the determination of the velocity of a ship and for velocity damping an inertial navigation system.
2. Background Art Statement
The Doppler sonar velocity log makes a relative velocity measurement using the Doppler shift effect at a single depth. This information is then sent to the inertial navigation system of the ship for use in the reduction of stability errors which are produced by changes in the motion of the ship.
Attempts to apply Doppler velocity log corrections in real time were unsuccessful. The problem with a single ping doppler velocity measurement in real time that is based on a single relatively small water column slice or bin is that it is subject to noise spikes which can cause erroneous velocities to be measured. Prior art systems depended on time averaging these single bin doppler velocity measurements to filter out these noise spikes. However, when performing the measurements this way, prior art systems filtered out true ship velocity changes because the long term averaging, on the order of ten to twenty seconds, smooths out interim changes.
In addition, the single bin measurements of Doppler velocity taken close to the hull of a ship produced system errors due to the ship effects such as bubble sweepdown, turbulent boundary layer and hull displacement effects on the water column bin. Additionally, errors related to ocean surface disturbances due to wind such as sea state influences were produced. The single bin of the prior art systems was typically fifteen meters or less below keel. The analog phase locked loop method of calculation used by the Doppler sonar velocity log was firmware controlled. The doppler produced reference velocity errors in the navigation damping loop of the Ships Inertial Navigation System (SINS) due to these factors were uncompensated.
Other state of the art systems employing correlation sonar and electromagnetic logs input velocity errors due to these same factors into the navigation systems which are also uncompensated.